1. Field of the Invention
This invention relates generally to cable television communication systems. More particularly, the invention relates to a method and apparatus for selectively permitting access to cable television communication system services.
2. Description of the Related Art
A number of different types of cable television (CATV) security systems exist to block premium or pay channel signals to those subscribers unwilling to pay for the various services. One of the more popular methods of signal scrambling is based on sync suppression. Located at the headend of a CATV system, video processing circuits suppress the horizontal synchronizing pulse from the video signal. The sync information is obscured from the sync separator of a television receiver by relocating it within the transmitted picture information. Since television receivers are unable to process a synchronization suppressed signal with the horizontal pulse relocated, the signal is scrambled. Subscribers desiring to view the scrambled channels must have a CATV settop terminal which is programmed to activate circuits to restore the suppressed horizontal synchronizing signal.
A simpler alternative to provide premium channel security is through the use of negative and positive trapping filters. The negative signal trap includes a network of passive electronic components, such as capacitors and inductors, in the form of a notch filter which is connected in series with a subscriber's CATV drop line. The filter removes a selected channel supplied to a subscriber's home. Negative signal traps are usually mounted outside of a subscribers' home on a utility pole nearby the cable tap. This method of channel security is the most secure since to defeat the trap requires access to the CATV plant located outside of a subscriber's home and would require climbing a utility pole.
However, this method of policing CATV has become unmanageable with the growing number of available channels in CATV communication systems. A negative type signal trap is required for each channel to be secured. The cost of using negative trapping filters is prohibitive when only a small number of subscribers desire the secure channels since a separate trap must be installed on the drop line of every subscriber not authorized to receive that channel.
Positive signal trapping injects an interfering signal in the secured channel carrier at the headend of a CATV system to render the programming on that channel unviewable. Authorized subscribers are given narrow band rejection filters which attenuate the interfering signal to restore the secured channel. Although the positive traps are inexpensive, the method is easy to defeat since all that is required to receive a clear picture is a notch filter of the correct frequency which are easy to obtain.
In an effort to eliminate the local filtering pitfalls, interdiction systems which introduce an interfering signal local to the subscriber are sometimes used. Typically, the system includes a plurality of a pole-mounted devices, each of which serves a minimum of four subscribers. The interdiction devices contain an authorization module incorporating address-recognition circuitry which monitors the downstream broadband signal to decode signals representing a unique subscriber's address and channel authorization information to selectively switch on or move jamming oscillators from channel to channel. The interfering or jamming signal is injected into a particular subscriber's unauthorized channels to confuse the television receiver's detector circuitry. Although a settop terminal or a cable-ready television receiver which can tune mid and superband CATV channels is required, no descrambler is needed.
Interdiction systems usually employ a frequency agile jamming oscillator which is used for a plurality of different carrier frequencies on a time division basis. The jamming signal must have high accuracy and temperature tolerances so as not to deviate from the desired carrier frequencies of the unauthorized channels. In real practice, unwanted artifacts are manifested in other regions of the transmission, thereby degrading overall reception quality to a given subscriber. Additionally, oscillator leakage may degrade the quality of signals to neighboring subscribers.
The most recent development in channel security is the use of a microprocessor within a settop terminal coupled with a receiver and address recognition circuitry. The settop terminal is addressable such that the CATV headend directly communicates with an individual subscriber. In this manner, the subscriber's CATV settop terminal observes a dedicated carrier carrying addressing data where it accesses encoded instructions specific to a secured channel and updates the settop terminal to permit viewing.
Unfortunately, all prior art methods for signal security present significant drawbacks of expense and technical problems for the cable provider. There is a need for an improved and inexpensive program deprivation unit regardless of the type of settop terminal or the sophistication of the television receiver used.